Radio transmitters in most telecommunications systems are required to accurately reproduce a signal present at the input of the radio transmitter at an output amplifier of the transmitter. An output amplifier that compresses its input or has a non-linear input-to-output relationship causes the output signal to distort and leak onto adjacent radio channels, resulting in undesired interference. Digital predistortion (“DPD”) circuits inversely model an amplifier's characteristics and when combined with the amplifier, produce an overall system that is more linear and reduces the amplifier's distortion by attempting to cancel any non-linearity the amplifier might have.
Predistortion is used in particular in high power radio transmitters, as the amplifiers in these transmitter tend to become more non-linear as their output power increases towards a maximum allowable output. State of the art high power transmitters implement amplifier linearization techniques, such as digital predistortion, to increase power and efficiency while meeting the adjacent channel leakage limits specified by wireless standards.
In traditional systems using predistortion techniques, transmitter baseband bandwidth commonly may be at least five times greater than that of the desired transmit signal. In these systems, noise associated with a transmit synthesis band can fall into or overlap a paired receive band, which limits the sensitivity of the receiver. This may also result in undesired emissions due to distortion. Efforts to address the undesired noise in the receive band have focused on cavity filters that are grouped in duplexers to reject the receive band noise. These cavity duplexers allow for the passing through of desired frequencies and rejection of the undesired frequencies, in this case, rejecting the receive band noise at certain noise levels. Unfortunately, cavity duplexers are difficult to implement because of the high cost of cavity filters, rendering these systems expensive from a cost standpoint.
Thus there remains a need in the art for an efficient, cost-effective radio transmitter with digital predistortion having reduced noise from the receive band in the transmit synthesis bandwidth. There further remains a need in the art for a process for reducing noise in a transmit synthesis bandwidth in a radio transmitter.